Conditional Type Checks


As TypeScript's type system becomes more complex, it's useful to be able to write tests for what a type should be.

This library offers reusable conditional types to help test your types.

Type Checks

These will resolve to the type true when they match and false otherwise.

- IsNullable - Checks if T is possibly null or undefined.
- IsExact - Checks if T exactly matches U.
- Has - Checks if T has U.
- NotHas - Checks if T does not have U.
- IsAny - Checks if T is the any type.
- IsNever - Checks if T is the never type.
- IsUnknown - Checks if T is the unknown type.
- More to come...

Ways to Test

Use what you prefer:

1. The AssertTrue, AssertFalse, or Assert types.
2. The assert function.

$3

Doing a test:

``ts
import type {
AssertFalse,
AssertTrue,
Has,
IsNever,
IsNullable,
} from "https://deno.land/x/conditional_type_checks/mod.ts";

const result = someFunction(someArg);

type _test =
| AssertTrue | IsNullable>
| AssertFalse>
| Assert, true>;
`

Warning: Do not use an intersection type between checks (ex. Has & IsNever) because it will cause everything to pass if only one of the checks passes.

$3

Doing a test:

`ts
import {
assert,
IsExact,
} from "https://deno.land/x/conditional_type_checks/mod.ts";

const result = someFunction(someArg);

// compile error if the type of result is not exactly string | number
assert>(true);
`

Failure:

`ts
// causes a compile error that true is not assignable to false
assert>(true); // string is not nullable
`

npm Install

`
npm install --save-dev conditional-type-checks
``